1. Field of the Invention
The present invention relates to an interdigital filter, and more specifically to an interdigital band-pass filter comprising a plurality of resonators coupled in the even and odd modes of a transverse electromagnetic wave.
2. Description of the Prior Art
Interdigital band-pass filters have been utilized in transmitters, for example, because of the high quality factor Q thereof. Such an interdigital filter utilizes a plurality of resonators coupled to each other not in the harmonic modes but in the even odd modes of the transverse electromagnetic wave.
FIGS. 1A and 1B show an example of a prior art interdigital band-pass filter, wherein FIG. 1A shows a plan view of the filter with a cover removed and FIG. 1B shows a sectional view of the filter taken along the line IB--IB. Such prior art filters may be seen, for example, in pages of the book entitled "Microwave Filters, Impedance-Matching Networks, and Coupling Structures" published by Mcgraw-Hill Book Company. Referring to FIGS. 1A and 1B, upper and lower conductor plates 1 and 2 are kept in parallel with each other spaced apart from each other by the distance H. The upper and lower conductor plates 1 and 2 may be made of a metal plate such as an aluminum plate and serve as a ground conductor. Metal spacers 1a are provided on the lower surface of the upper conductor plate 1 at both sides i.e. the upper and lower sides of the upper conductor plate, as viewed in FIG. 1A and metal spacers 2a are provided on the upper surface of the lower conductor plate 2 at both sides, i.e. the upper and lower sides of the lower conductor plate 2 as viewed in FIG. 1A. A plurality of resonant conductor rods 3, 3, 3 . . . are provided between the metal spacers 1a and 2a so as to extend alternately from either side of the conductor plates 1a and 2a in the transversal direction of the conductor plates 1 and 2 with a predetermined distance d from each other. The length of the resonant conductor rods 3, 3, 3 . . . is selected to be l which is shorter than the width L between the spacers at both sides. As a result, a cut-off space 9 is formed between the terminal end of the resonant conductor rod 3 and the metal spacers in the opposite side of the conductor plates 1a and 2a. Thus, the resonant conductor rod 3, 3, 3 . . . are arranged in the so-called interdigital manner, as seen in FIG. 1A. Referring further to FIG. 1A, an input coupler 41 is provided in parallel with and in the vicinity of the left end resonant conductor rod 3 as viewed in FIG. 1A, while an output coupler 42 is provided in parallel with and in the vicinity of the right end resonant conductor rod 3 as viewed in FIG. 1A. The input coupler 41 is coupled to an input coaxial connector 51 through an impedance matching terminal, while an output coupler 42 is coupled to an output coaxial connector 52 through an impedance matching terminal. Such an arrangement is packed to provide a complete interdigital filter 10. As is well known, the resonant conductor rods 3, 3, 3 . . . are coupled to each other in the even and odd modes of the transverse electromagnetic wave in such an interdigital filter 10. As a result, the interdigital filter 10 exhibits a resonance characteristic as shown in FIG. 2, wherein the ordinate shows an attenuation and the abscissa shows the frequency.
Such is interdigital filter 10 as described in the foregoing was not able to be made compact, because the distance H, the width L and the length W were not able to be made small due to a restriction to a requirement in terms of the characteristics of the filter. Generally, it is required that such a filter be of a high quality factor Q which makes it difficult to make the effective distance H smaller than a predetermined value, inasmuch as a decreased distance H decreases the quality factor of the filter. In addition, if the distance d between the adjacent resonators and thus the resonant conductor rods 3 becomes too small, the degree of mutual coupling of the resonators become too large, which makes too broad the band width of the frequency characteristic of the filter. Furthermore, the width L is restricted because of the inherent length l of the resonant conductor rod 3 and the cut-off space at the open end of the resonant conductor rod. A filter of a narrow band width could be provided by decreasing the degree of the mutual coupling between the resonant conductor rods. In such a situation, however, it is necessary to increase the distance d and thus the length W, which degrades a temperature characteristic although the quality factor remains high. More specifically, it could happen that if the band width is made narrow the central frequency fo could vary greatly by virtue of the temperature variation. Thus, in spite of a demand for a compact interdigital filter, there has been difficulty in miniaturizing such a prior art interdigital filter. Accordingly, this difficulty to in miniaturizing of such interdigital filters has been a hindrance to compactness of the whole system where such interdigital filter is utilized.